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In electrophotographic printing of images such as in the xerographic process, toner is attracted to a surface of a photoconductive drum or belt selectively charged and then transferred by electrostatic processes to print output media such as paper. The photoconductive drums or belts must function to hold a selectively applied charge corresponding to a document or image original. The output media must be able to dielectrically relax appropriately in order for fast, efficient printing of high quality images to be produced.
The properties of these materials that are so critical in the production of electrophotographic images have been poorly understood for the intended use. Traditional characterization of these materials typically involved resistance measurements and simple RC modeling, which was found to be of limited use if at all in selecting or designing materials advantageous for such uses.
Special reproduction environments present additional problems the causes of which are not well understood. These include the presence of toner scatter or print dropouts accompanying electrostatic discharge in the air gap over the paper, the potential for image deletion or image print through in duplex printing due to the presence of an image on the reverse side in the second pass through, the tendency of transparency material to not support good images printed on it and the problem of color shift due to residual charge effects in multi-pass color printing. Therefore, a novel technique for characterization of these materials for the purposes of understanding the mechanism, predicting the performance, and specification for material design, is needed.
The present invention tests materials for use in electrophotograpic printing with a whole set of characteristics determined to be important for efficiency and high quality images. The materials include, but not limited to, photoconductive drums or belts, charging rolls, developer rolls, intermediate transfer belts and output media such as paper, transparencies or textiles. It is well known that the performance of these materials in this application depends critically on the process of dielectric relaxation when the material is under electrical stress. The traditional method for electrical characterization of these materials typically involves measurements of resistance in closed-circuit experiments and analyses based on the equivalent circuit model, which have been found to be of limited success in correlating the results with the imaging performance.
In the present invention, a test system and data analysis procedure are provided to characterize dielectric relaxation process in these materials in terms of charge transport parameters that include, but not limited to, intrinsic charge density, charge mobility, and charge injection from the contact surfaces.
The apparatus of this invention consists of a charging source, a voltage detector and a current detector in an open-circuit mode of measurement. The configuration closely simulates the actual application of the materials in electrophotography and thus, can yield information more relevant for the applications (than the conventional resistance measurement in a closed circuit). Furthermore, the non-contact feature of the test system enables non-destructive, high speed scanning evaluation over a large two-dimensional area of the material. The apparatus is in part similar to the one described in commonly owned U.S. Pat. No. 5,929,640, incorporated herein by reference.
The data are processed based on a model developed from first principle charge transport theory. The model provides the procedure for deducing the above-mentioned charge transport parameters from the measured voltage and/or current. Furthermore, a single figure of merit, namely, an effective resistance or an apparent resistance, that consolidates the roles of a large number of charge transport parameters mentioned above, can also be deduced from the measured voltage and/or current for routine characterization such as in production quality control.
The data acquisition and processing described above are carried out automatically by the control software.
A good correlation has been obtained between the charge transport parameters and printing performance. Common print quality deficiencies, for example, toner scatter, image deletion, image print through in duplex printing, and color shifts in full color prints, can be attributed to inadequate dielectric relaxation in the materials involved, in this case, the transfer media. Thus, a dielectric relaxation profile of a material obtained by the technique and analysis of this invention serves the purpose of performance prediction and design guideline for new materials.